This invention relates to a fluidized bed reactor, and, more particularly, to such a reactor in which heat is generated by the combustion of fuel in a fluidized bed.
Fluidized bed reactors, combustors, or gasifiers, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. When the heat produced by the fluidized bed is utilized to convert water to steam, such as in a steam generator, the fluidized bed system offers an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a bubbling fluidized bed in which a bed of particulate materials is supported by an air distribution plate, to which combustion-supporting air is introduced through a plurality of perforations in the plate, causing the material to expand and take on a suspended, or fluidized, state. In the event the reactor is in the form of a steam generator, the walls of the reactor are formed by a plurality of heat transfer tubes. The heat produced by combustion within the fluidized bed is transferred to a heat exchange medium, such as water, circulating through the tubes. The heat transfer tubes are usually connected to a natural water circulation circuitry, including a steam drum, for separating water from the steam thus formed which is routed to a turbine to generate electricity or to a steam user.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a fast, or circulating, fluidized bed. According to this technique, fluidized bed densities ranging to 20% volume of solids are attained which is well below the 30% volume of solids typical of the bubbling fluidized bed. The formation of the low density circulating fluidized bed results from smaller particle sizes and higher fluidizing velocities. For a material balance a higher solids recycle is required. The velocity range of a circulating fluidized bed is between the solids terminal, or free fall, velocity and a velocity which is a function of the throughput, beyond which the bed would be converted into a pneumatic transport line.
The high solids circulation required by the circulating fluidized bed makes it insensitive to fuel heat release patterns, thus minimizing the variation of the temperature within the combustor or gasifier, and therefore decreasing the nitrogen oxides formation. Also, the high solids loading improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the required adsorbent addition. Furthermore, the circulating fluidized bed inherently has more turndown than the bubbling fluidized bed.
The circulating fluidized bed reactor, however, requires relatively large cyclone separators, which negates the possibility of a compact design which can be modularizied and easily transported and erected. This is a major disadvantage especially when the fluidized bed is utilized as a steam generator. Also, the particulate fuel and adsorbent material used in a circulating fluidized bed process must be relatively smaller in size therefore requiring further crushing and drying of the feedstock material, which is expensive. Further, in a circulating fluidized bed system, the bed height required for adequate adsorption of the sulfur is greater than that in a conventional bubbling fluidized bed system, which further adds to the capital expense and operating costs.